Optical position sensing systems, such as those used in connection with computer displays, office machinery, gaming equipment, etc., rely on a combination of line-scan or area image cameras, digital signal processing, front or back illumination and algorithms to determine a point of touch. Many optical position sensing systems use cameras, orientated along the touch screen surface so as to image the bezel. In this way, the system can track the movement of any object close to the surface of the touch screen by detecting variations in illumination emitted by an illumination source, such as an infrared light source.
While cameras generally are more expensive than other types of detector devices that can be used in optical position sensing systems, such as photo-detectors (e.g., photo-diodes or photo-transistors), they allow greater accuracy for touch detection. As known in the art, cameras using both area scan or line scan sensors are typically expensive and too large in the dimensions which are critical to commercially viable small touch screens.
Conventional optical position sensing systems use optical position sensors comprising multiple refractive elements (i.e., multiple element lens systems). Typically, these refractive elements are plastic or glass lenses. Lenses commonly used in optical sensors and other camera devices are typically designed for imaging applications. They are designed to have low image distortion when imaging a plane surface. Ideally, when light is transmitted and/or refracted onto a lens, all the rays of light are converged to a single point, resulting in a clear image. However, in most lens systems light rays are diverted to different points due to lens imperfections and other influences. These influences are commonly called aberrations, and usually result in distorted images.
Conventional camera devices use multi-element lens systems because the use of multiple refractive elements makes it possible to correct and compensate for aberrations and image distortion over a single element lens system, increasing the clarity of the image. However, the use of multiple elements increases the overall size of the camera, especially the depth, and makes it more difficult to converge light rays at a single point. The problem is exacerbated in configurations where space is extremely limited. While distortion may be undesirable for imaging applications, this is not the case for optical position sensing. Thus, conventional position sensing systems do not require the primary benefit of a multi-lens system. Further, the increased size of multi-lens camera systems not only adds unwanted space to the overall system, but it also adds to the expense of manufacturing these systems.
Additionally, when focusing the lens of conventional multi-element lens systems, manufacturers must physically move the lens elements relative to the body and sensor of the camera. Commonly this is done by a threaded lens barrel, and this results in a camera height which is set by the lens diameter. This is a difficult process given the relatively small amount of available space in a position sensing system. Additionally the multi element lenses and the focusing mechanisms are not mechanically robust, and sensitive to vibration. Unlike imaging applications, slight movement in the optical path causes significant position errors, even when no image degradation would result.
In a retroreflective system, the triangle formed between the illumination source, the nearest point of the reflector, and the lens aperture, must subtend an angle less than the observation angle of the retroreflective material. Existing systems use low performance reflective materials such as beaded material, which compromise performance, and large screen sizes, so that the observation angle is large. Other known systems use expensive beam splitting optics, which are extremely susceptible to dust and contaminants blinding the camera. It is an objective of this invention to use high performance reflective material, on small screens, without expensive beam splitting optics, and with advantageous immunity to blinding from dust and other contaminants.